The `Stream.render` now accepts an optional `out` parameter that can be used to pass in a writable file-like object to use for assembling the output, instead of building a big string and returning it.

<div id="header">
  <h1><?cs var:title ?></h1>
</div>

<?cs def:greeting(name) ?>
  <p>Hello, <?cs var:name ?>!</p>
<?cs /def ?>